1. Field of the Invention
The present invention relates to semiconductor laser apparatuses, and more particularly, to an arrayed Distributed Bragg Reflector (DBR)-semiconductor laser apparatus that realizes a tunable semiconductor laser capable of setting any optional wavelength.
2. Description of the Related Art
As tunable lasers capable of optionally setting a wavelength of the entire C-band of wavelength band used in a wavelength division multiplexer (WDM: Wavelength Division Multiplexer) system, DBR lasers having a special diffraction grating such as SG (Sampled Grating), SSG (Super Structure Grating) or the like have been developed. In order to perform laser oscillation at a desired wavelength, DBR lasers needed current control for optical phase adjustment in addition to current control for wavelength adjustment. The control system became complex and additionally it was hard to ensure long-term wavelength durability. In order to overcome the aforesaid problems, a short cavity DBR laser not requiring any optical phase adjustment has been recently developed (refer to “Selected Topics in Quantum Electronics”, IEEE Journal, Vol. 9, September/October of 2003, p. 1132-1137, for example). Since the variable range of wavelength per channel in this short cavity DBR laser is limited to 10 nm or less, it is necessary to array a plurality of DBR lasers for covering the entire C-band. As a form for realizing the foregoing arrangement, it has been known in the art to provide a semiconductor laser for combining DBR laser arrays through a multi-mode interferometer (MMI: Multi-Mode Interferometer) and amplifying it through a semiconductor optical amplifier (SOA: Semiconductor Optical Amplifier). FIG. 1 shows an example of a laser chip having the DBR laser arrays, MMI and SOA integrated in monolithic form. The laser chip is made such that gain electrodes 101, 102, 103 and 104, DBR electrodes 105, 106, 107 and 108 and SOA electrode 114 are formed on the surface of an InP substrate 100. A waveguide structure is such that a DBR laser channel ch1 115, a DBR laser channel ch2 116, a DBR laser channel ch3 117, and a DBR laser channel ch4 118 are arranged in parallel to one another, to each of which corresponding one of optical waveguides 109, 110, 111 and 112 is connected on the optical output side. The optical waveguides 109, 110, 111 and 112 are connected to an MMI multiplexer 113, which is further connected to an SOA waveguide below an SOA electrode 114. FIG. 2A is a plan view of the chip shown in FIG. 1. This laser chip includes a rear DBR region 138, a gain region 139, a front DBR region 140, an S-shaped waveguide region 141, an MMI region 142 and an SOA region 143, which are integrated therein. A front end surface 144 and a rear end surface 145 are coated with a low reflection film. FIG. 2B is an ABCDE-sectional view of the chip shown in FIG. 2A. In this case, the ABCDE-section is defined as sections of an optical path ranging from a semiconductor laser shown at the upper-most part in FIG. 2A to a semiconductor optical amplifier through the optical waveguides, a multiplexer for multiplexing the optical waveguides. The DBR laser part is made such that a core layer 132 in the gain region, a refractive index control core layer 134 in the rear DBR region and a refractive index control core layer 135 in the front DBR region are connected to one another. Refractive grating supplying layers 136 are disposed on the refractive index control core layer 134 in the rear DBR region and the refractive index control core layer 135 in the front DBR region. The refractive index control core layer 135 in the front DBR region is connected to a core layer 137 of the low loss optical waveguide, which forms the S-shaped waveguide region 141 and the MMI region 142. Further, the core layer 137 is connected to the core layer 133 in the SOA region.
FIG. 3 shows an example of the wavelength characteristic of the laser chip in FIG. 1. The DBR lasers of 4 channels, ch1, ch2, ch3 and ch4, covers wavelength regions different from one another so as to cover the entire C-band. In order to provide a wavelength of 1540 nm, for example, a current is inputted to the gain layer and SOA layer of ch2 and then a current of 100 mA is inputted to the DBR layer. At this time, no current is allowed to flow in the gain layer and DBR layer of the DBR laser with other channels.
However, when a current is allowed to flow to provide a sufficient optical output, a problem arises of degrading spectrum purity. FIG. 4 shows a spectrum obtained by operating the laser chip in FIG. 1. The laser chip is operated such that the DBR laser channel ch4 118 is selected, a current of 20 mA is inputted to the gain electrode 102, 5 mA to the DBR electrode 106 and 200 mA to the SOA electrode 114. In addition to a main signal 301 of 1560 nm, both background light 302 with a narrow wavelength range and background light 303 with a wide wavelength range are generated, thus, degrading spectrum purity. As a result, an intensity ratio between the main signal and the background light (here, referred to as an SNR: Signal Noise Ratio) is 35 dB, which does not satisfy 40 dB requisite for optical communications in general.
The background light 302 with a narrow wavelength range is probably due to the following: Spontaneous emission light occurring upon application of a current to the core layer 133 in the SOA region passes through the MMI region 142 and the S-shaped waveguide region 141. Then, it reaches the refractive index control core layer 135 in the front DBR region in each of the DBR laser channels. Each of the front DBR refractive index control layers 135 of the DBR laser channels reflects spontaneous emission light. The reflected light is returned back to the core layer 133 in the SOA region, amplified there and then the amplified light is output from the end surface 144. In addition, the background light 303 with a wide wavelength range is probably due to the following: Intensity of signal light inputted to the core layer 133 in the SOA region is weak because signal light generated by the DBR laser 118 is lost at the MMI. This leads to a large amount of current not used for amplifying the inputted light. This surplus current generates the background light 303 with a wide wavelength range.